The present invention is directed to power control systems for electric traction motor propelled vehicles and, more particularly to safety and protection apparatus for use with such systems.
In many electric vehicles, electronic power regulators are used to control the torque, or speed, developed by electric traction motors propelling the vehicle. Typically, the regulator comprises a time-ratio or chopper circuit which varies the power developed by the motors by controlling the percentage of time that the motors are connected to a power source. The chopper circuit may be either a transistor or a thyristor (SCR) circuit. In either case a mechanical bypass switch is generally connected in shunt with the chopper circuit. The bypass switch is closed for high conduction ratio (high current) of the chopper circuit to minimize thermal stress on the solid state components.
The vehicle system also includes a series contactor which may be activated by a key switch for connecting the regulator and motor to the power source.
It is known for the contact tips of the bypass contactor to fail in a closed state, e.g., the tips may be welded together by a sudden current surge at closure. It is also known for the chopper circuit to fail in a shorted condition. If an attempt is made to start a vehicle in which either of these failures exist, large currents are developed in the motors upon closure of the series contactor. Although a series connected DC motor is somewhat protected by the relatively high impedance of the field winding, the armature of a separately excited motor is not so protected. In fact, in an 84 volt system, armature current can quickly jump to over 3000 amperes causing severe damage to the armature.
Of even more concern, however, is the possibility of having the main contactor weld closed due to a failure of the bypass switch or the chopper circuit. This type of failed condition could result in a runaway vehicle.
It is an object of the present invention to provide a protection system for use in a DC electric motor power control system.
It is a further object of the invention to provide a protection system which will detect a failed condition of a power regulator or a bypass contactor and inhibit operation of the control system.